1. Field of the Invention
The invention pertains to a lock-up clutch with at least one friction area on a first converter component, which can be shifted into working connection with at least one opposing friction area on a second converter component by an engaging movement, or which can be separated from this opposing friction area by a disengaging movement in the direction opposite that of the engaging movement.
2. Description of the Related Art
U.S. Pat. No. 5,215,173 discloses a lock-up clutch for a hydrodynamic torque converter with a piston, which, as shown in FIGS. 1 and 2, carries a friction lining on the side facing a converter cover; the side of this lining which faces away from the piston serves as a friction area. The piston can be moved toward the converter cover to engage the lock-up clutch or away from it to disengage the clutch. As soon as the friction area of the friction lining makes contact with the converter cover, the corresponding contact surface of the latter serves as the opposing friction area. The piston serves as the first converter component, and the converter cover serves as the second converter component of the lock-up clutch.
As soon as the friction lining of the piston comes into contact with the converter cover the rotational movement of the converter housing around its rotational axis is no longer transmitted to the transmission input shaft via a hydrodynamic circuit with an impeller, a turbine wheel, and a stator; instead, the movement is transmitted directly to the shaft by means of the lock-up clutch.
The use of a lock-up clutch may be advantageous from the standpoint of energy efficiency, but in this operating state the lock-up clutch should be used to damp the torsional vibrations which may have been introduced along with the torque. For this reason, the piston of the lock-up clutch is connected by a torsional vibration damper to the transmission input shaft; the torsional vibration damper has stored-energy elements to absorb elastically the torsional vibrations. In designs of lock-up clutches without torsional vibration dampers, such as that shown in FIGS. 3 and 4 of the same Offenlegungsschrift, however, the pressure which presses the piston against the converter cover is reduced so that the piston can make a desirable slipping movement. Although this slippage may serve effectively to damp the introduced torsional vibrations, it allows in return a considerable amount of heat to build up in the friction area and in the opposing friction area. This principle of lock-up clutch operation with controlled slippage can be realized both with a so-called single-WK (single torque circuit) design according to FIGS. 1 and 2 and also with a double-WK design according to FIGS. 3 and 4, these two designs differing from each other only with respect to the number of friction linings. In the case of the double-WK design, however, the friction linings are preferably attached nonrotatably but with freedom of axial movement to a clutch disk located axially between the converter cover and the piston.
Regardless of the number of friction linings and accordingly of the number of friction areas and opposing friction areas, the previously mentioned buildup of heat caused by slippage makes it necessary to take measures to ensure that this heat can be carried away as quickly as possible from the working area of the lock-up clutch. In U.S. Pat. No. 5,215,173, this is done by thermal conduction through the metal as a result of the temperature gradient which exists between the origination point of the heat and the other areas of the hydrodynamic torque converter through which hydraulic fluid is flowing. When large amounts of frictional work are performed, however, this type of cooling is no longer sufficient, which means that the friction linings will become overheated and that the hydraulic fluid passing over them will deteriorate.
U.S. Pat. No. 5,575,363 discloses an elaboration in this regard. FIGS. 14-17 in particular show systems of grooves either in the friction linings or at least in one of the two converter components, i.e., in the converter cover or in the piston. These grooves make it possible for hydraulic fluid to flow from the radially outer area toward the radially inner area. As shown in FIG. 1 of this patent, the hydraulic fluid can be carried away through channels provided for the purpose to the rotational center of the hydrodynamic torque converter and thus conveyed out of the converter circuit. The disadvantage, however, is that the grooves offer relatively high resistance to the flow of the fluid; this can be caused, first, by comparatively small flow cross sections of the grooves and, second, by long travel distances in the grooves. A high pressure difference must therefore be built up so that a sufficiently high volume flow rate of hydraulic fluid through the grooves can be obtained in spite of the previously mentioned high flow resistance. As a result, lifting forces which try to lift the piston also develop axially between the converter cover and the piston, and to counteract them it is necessary to apply higher contact pressures. Precisely when high torques are being transmitted, therefore, a considerable amount of energy must be expended to maintain this cooling process.
It is also extremely difficult to lay out the flow cross sections and the lengths of these grooves in such a way that the precise pressure difference required to force the hydraulic fluid through them is obtained. An advantageous solution to this problem is described in U.S. Pat. No. 5,732,804, according to which the length of the grooves is reduced and their cross sections are made relatively large. At least one throttle point is provided, which can be used to adjust the volume flow rate. This throttle point preferably passes through at least one of the friction linings in the axial direction. But even this solution with its advantageous engineering design still provides only a limited cooling action.
Systems of grooves in the friction lining have also become known in which each individual groove has both its inflow and its outflow points on the same radial side of the friction lining, whereas the friction lining also has a friction area radially outside these grooves. This radially outer friction area is closed in the circumferential direction and is intended to prevent a possible pressure-induced leak-through of hydraulic fluid from the radially outer area inward toward the radially inner area. This design of a groove system, shown by way of example in U.S. Pat. No. 4,986,397, may indeed reduce any tendency of the piston to be lifted from the converter cover assigned to it when the pressure of the fluid flowing through the grooves builds up, but on the other hand it suffers from the disadvantage that the flow through each individual groove occurs exclusively by reason of shear forces in the hydraulic fluid, these shear forces being caused by the relative velocity between the limiting surfaces. The conveyed volume flow rate is therefore low, and, in addition, some of the hot hydraulic fluid forced out of the groove which precedes in the circumferential direction is taken up again by the groove which follows in the circumferential direction. The cooling effect which can be achieved is thus correspondingly low.
The invention is based on the task of designing the friction area of the lock-up clutch of a hydrodynamic torque converter in such a way that a highly effective cooling action can be obtained in the friction area while good energy efficiency is provided at the same time.
This task is accomplished according to the invention by providing a pump on at least one of the converter components, the pump acting in the friction area or in the opposing friction area, namely, a pump with a geometric design which allows it to generate a pressure gradient, it is possible to force the flow of hydraulic fluid through at least one predetermined section, of the friction area and/or of the opposing friction area.
Thus the pump device can have a baffle element, for example, which projects into the flow path of the hydraulic fluid and causes turbulence to develop in at least certain parts of the fluid. As a result of this turbulence, the hydraulic fluid which has just flowed through the circumferentially preceding groove and thus become hot is mixed much more effectively with other hydraulic fluid which is present a greater distance away from the friction lining and is correspondingly cooler. Because of this intensive mixing, a hydraulic fluid is obtained which, by reason of its temperature, is able to carry away considerable amounts of heat from the circumferential following groove as the fluid flows through the grooves. In addition, because of the baffle element arranged in the flow path of the hydraulic fluid, it is ensured that a comparatively high volume flow rate is directed into the groove, so that, in the inflow area of the groove, a considerable dynamic pressure is built up, as a result of which the volume flow rate through the groove is significantly increased. This effect can be increased even more by providing the inflow to the groove with a cross section larger than that of the rest of the groove, so that the volume flow rate of the newly arriving, cooler hydraulic fluid quickly forces the heated hydraulic liquid already in the groove out of the groove. If this inflow to the groove is also made as free of edges as possible, the problem of a possible separation of the arriving flow can be reduced, and the turbulence can be increased in the inflow area of the groove as well.
The function of a pump device can also be assumed by a displacement body, provided on at least one of the two converter components in the area where the one converter component comes in contact with the groove in the other converter component, this displacement body fitting at least partially into this groove, thus narrowing its flow cross section. As a result of the relative motion between the two above-mentioned converter components in the circumferential direction with respect to each other, the displacement body has the effect of producing the desired forced flow through the groove, where, as a result of the flow resistance in the groove, the pressure reaches a maximum at a point which is immediately ahead of the displacement body in terms of the direction of motion and then decreases to a minimum behind the displacement body. Through the proper selection of the cross section of the grooves, the flow resistance in the groove system can be adjusted in such a way that a part of the flow volume is forced out of the groove system, while fresh hydraulic fluid is drawn in on the other side. Obviously, this inventive solution is not limited to an individual displacement body; on the contrary, it can be enhanced by the use of several such displacement bodies.
Another way in which the pump device can be designed involves the creation of an eccentricity between the friction area and the opposing friction area and to enclose this eccentricity area with a seal extending radially around the outside. Relative movement between the friction area and the opposing friction area thus leads to the formation of a revolving displacer volume, so that every possible point of the friction area and of the opposing friction area is, wetted with hydraulic fluid during each revolution, as a result of which a superior cooling effect can be obtained. An especially characteristic feature of this solution is that the friction area and the opposing friction area approach each other to form a point of minimum separation distance. The pressure differences required for this principle to work are created in front of and behind this point, which is called the dynamic pressure point; in a preferred embodiment, these differences can be used to generate a volume flow rate through a lining provided with grooves. It is especially preferable for this system of grooves to have a zigzag or meander-shaped form or to have a waffle-shaped groove system. Because of the previously mentioned revolving wetting process, however, it is not absolutely necessary for the sake of the above-mentioned principle that the friction area or the opposing friction area be formed in one of these ways.
The previously mentioned point where the minimum radial distance is present between the friction area and the opposing friction area can be provided on the radially outer side of the friction linings, but it is equally possible to provide it on the radially inner side or even on both radial sides. For this purpose, preferably an area projecting in the axial direction is provided on the converter component without a friction lining, this axial extension enclosing the radially adjacent friction lining radially from the outside or approaching it radially from the inside. A design is conceivable here in which each of the axial extensions is in the form of a ring, whereas the associated friction lining has an elliptical shape. Nevertheless, this type of design variant should be limited to the use of lock-up clutches which have an even number of friction linings, so that, by arranging them with an angular offset with respect to each other, they can compensate for radial vibrations caused by the elliptical shape of the friction linings and thus successfully avoid any balance errors which may otherwise occur.
Exemplary embodiments of the invention are explained in greater detail below on the basis of a drawing.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.